Switch device with a wide bandwidth

A switch device includes a common terminal and a selection circuit. The selection circuit includes a primary switch, a first secondary switch, and a second secondary switch. The primary switch includes a plurality of primary transistors coupled in series and is coupled to the common terminal. The first secondary switch is coupled to the primary switch and a first transmission terminal. The first secondary switch includes a plurality of first secondary transistors coupled in series. The second secondary switch is coupled to the primary switch and a second transmission terminal. The second secondary switch includes a plurality of second secondary transistors coupled in series. The number of the first secondary transistors and the number of the second secondary transistors are both greater than or equal to the number of the primary transistors.